U.S. patent application number 16/942244 was filed with the patent office on 2021-03-11 for electric cable with improved thermal conductivity.
The applicant listed for this patent is NEXANS. Invention is credited to Dimitri CHARRIER, Christelle MAZEL, Daphne MERLE, Gabriele PEREGO.
Application Number | 20210074451 16/942244 |
Document ID | / |
Family ID | 1000005276316 |
Filed Date | 2021-03-11 |
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United States Patent
Application |
20210074451 |
Kind Code |
A1 |
PEREGO; Gabriele ; et
al. |
March 11, 2021 |
Electric cable with improved thermal conductivity
Abstract
A cable is provided having at least one electrically insulating
layer obtained from a polymer composition with at least one
polypropylene-based thermoplastic polymer material and at least one
inorganic filler selected from aluminium oxide, a hydrated
aluminium oxide, magnesium oxide, zinc oxide, and a mixture
thereof; and a method for making the cable.
Inventors: |
PEREGO; Gabriele; (Milan,
IT) ; MAZEL; Christelle; (RUY, FR) ; CHARRIER;
Dimitri; (Ecully, FR) ; MERLE; Daphne;
(VENISSIEUX, FR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NEXANS |
Courbevoie |
|
FR |
|
|
Family ID: |
1000005276316 |
Appl. No.: |
16/942244 |
Filed: |
July 29, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01B 7/0275 20130101;
H01B 7/428 20130101; C08K 3/22 20130101; H01B 3/441 20130101; C08K
2201/005 20130101; H01B 13/14 20130101; C08L 23/14 20130101; C08L
2205/025 20130101; C08K 2003/2227 20130101; C08K 2201/001 20130101;
C08L 2207/02 20130101 |
International
Class: |
H01B 7/42 20060101
H01B007/42; H01B 3/44 20060101 H01B003/44; H01B 13/14 20060101
H01B013/14; H01B 7/02 20060101 H01B007/02; C08K 3/22 20060101
C08K003/22; C08L 23/14 20060101 C08L023/14 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 30, 2019 |
FR |
19 08652 |
Claims
1. Electric cable comprising: at least one elongated electrically
conducting element and at least one electrically insulating layer
obtained from a polymer composition comprising at least one
polypropylene-based thermoplastic polymer material and at least one
inorganic filler, wherein the inorganic filler is selected from
aluminium oxide, a hydrated aluminium oxide, magnesium oxide, zinc
oxide, and a mixture thereof.
2. Electric cable according to claim 1, wherein the inorganic
filler is aluminium oxide.
3. Electric cable according to claim 1, wherein the polymer
composition comprises at least 1 wt % of inorganic filler, relative
to the total weight of the polymer composition.
4. Electric cable according to claim 1, wherein the inorganic
filler is in the form of particles ranging in size from 0.01 to 6
.mu.m.
5. Electric cable according to claim 1, wherein the
polypropylene-based thermoplastic polymer material comprises a
propylene copolymer P.sub.1.
6. Electric cable according to claim 5, wherein the propylene
copolymer P.sub.1 is a random propylene copolymer or a heterophase
propylene copolymer.
7. Electric cable according to claim 1, characterized in that the
polypropylene-based thermoplastic polymer material comprises a
random propylene copolymer and a heterophase propylene copolymer,
or two different heterophase propylene copolymers.
8. Electric cable according to claim 5, wherein the
polypropylene-based thermoplastic polymer material further
comprises an olefin homopolymer or copolymer P.sub.2.
9. Electric cable according to claim 1, wherein the polymer
composition further comprises a dielectric liquid.
10. Electric cable according to claim 1, wherein the electrically
insulating layer is a non-crosslinked layer.
11. Electric cable according to claim 1, wherein the electrically
insulating layer has a tensile strength before or after ageing of
at least 8.5 MPa.
12. Electric cable according to claim 1, wherein the electrically
insulating layer has an elongation at break before or after ageing
of at least 250%.
13. Electric cable according to claim 1, wherein said electric
further comprises: at least one semiconducting layer surrounding
the elongated electrically conducting element, and at least one
electrically insulating layer surrounding the elongated
electrically conducting element, the electrically insulating layer
being as defined in any one of the preceding claims.
14. Method of manufacturing an electric cable as defined in claim
1, wherein said method comprises at least one step 1) of extrusion
of the polymer composition around the elongated electrically
conducting element, to obtain an electrically insulating layer
surrounding said elongated electrically conducting element.
Description
RELATED APPLICATION
[0001] This application claims the benefit of priority from French
Patent Application No. 19 08652, filed on Jul. 30, 2019, the
entirety of which is incorporated by reference.
FIELD OF THE INVENTION
[0002] The invention relates to a cable comprising at least one
electrically insulating layer obtained from a polymer composition
comprising at least one polypropylene-based thermoplastic polymer
material and at least one inorganic filler preferably selected from
aluminium oxide, a hydrated aluminium oxide, magnesium oxide, zinc
oxide, and a mixture thereof; and a method for making said
cable.
[0003] The invention applies typically but not exclusively to
electric cables intended for power transmission, notably
medium-voltage power cables (notably from 6 to 45-60 kV) or
high-voltage power cables (notably above 60 kV, and which may be up
to 400 kV), whether direct-current or alternating-current, in the
fields of overhead, underwater, or terrestrial electricity
transmission, or else aeronautics.
[0004] The invention applies in particular to electric cables
having improved thermal conductivity.
DESCRIPTION OF RELATED ART
[0005] A medium-voltage or high-voltage power transmission cable
preferably comprises, from the interior to the exterior: [0006] an
elongated electrically conducting element, notably made of copper
or aluminium; [0007] an internal semiconducting layer surrounding
said elongated electrically conducting element; [0008] an
electrically insulating layer surrounding said internal
semiconducting layer; [0009] an external semiconducting layer
surrounding said insulating layer, [0010] optionally an electrical
screen surrounding said external semiconducting layer, and [0011]
optionally an electrically insulating protective sheath surrounding
said electrical screen.
[0012] The electrically insulating layer generally comprises at
least one polyolefin such as an ethylene polymer (i.e. a homo- or
copolymer of ethylene), crosslinked or non-crosslinked. The
non-crosslinked ethylene polymers (e.g. non-crosslinked low-density
polyethylene or non-crosslinked LDPE) cannot generally be used at
temperatures above 70.degree. C., and therefore reduce the cable's
capacity to transport electrical energy in such a way as to avoid
any overheating of the electrically insulating layer at
temperatures above 70.degree. C. Conversely, crosslinked ethylene
polymers (e.g. XLPE) may be used up to temperatures of 90.degree.
C. However, these polymers are not easily recyclable, the
crosslinking (vulcanization) process for producing a homogeneous
layer is restrictive in terms of production cost, and/or production
capacity. Finally, crosslinking may sometimes start prematurely in
the extruder (screw, heater band) and/or the extruder head, leading
to formation of particles of degraded XLPE in the extruder (also
called "scorch"), which may then migrate into the electrically
insulating layer or into the semiconducting layer of the cable and
create defects there. The presence of these particles then affects
the final properties of the cable. This phenomenon is known by the
English name "scorch phenomena".
[0013] The propylene polymers generally have properties of thermal
conductivity slightly below those of the ethylene polymers.
Consequently, their use may lead to a decrease in removal of the
heat generated by the Joule effect, and thus in the amount of
energy transported, the latter being a function of the maximum
acceptable temperature of the elongated electrically conducting
element.
[0014] To overcome this problem, international application
WO2018167442A1 describes an electric cable comprising at least one
elongated electrically conducting element and at least one
electrically insulating layer obtained from a polymer composition
comprising at least one polypropylene-based thermoplastic polymer
material and at least one inorganic filler such as kaolin or chalk.
However, the mechanical properties of the cable thus obtained are
not optimized.
OBJECTS AND SUMMARY
[0015] The aim of the present invention is consequently to overcome
the drawbacks of the techniques of the prior art by proposing an
electric cable, notably medium-voltage or high-voltage, based on
propylene polymer(s), said cable, which is able to operate at
temperatures above 70.degree. C., having improved mechanical
properties, notably in terms of elongation at break and tensile
strength, while guaranteeing good thermal conductivity.
[0016] This aim is achieved by the invention that will be described
below.
[0017] The first object of the invention is an electric cable
comprising at least one elongated electrically conducting element
and at least one electrically insulating layer obtained from a
polymer composition comprising at least one polypropylene-based
thermoplastic polymer material and at least one inorganic filler,
characterized in that the inorganic filler is notably a metal
oxide, whether or not hydrated, said inorganic filler preferably
being selected from aluminium oxide, a hydrated aluminium oxide,
magnesium oxide, zinc oxide, and a mixture thereof.
[0018] The combination of a polypropylene-based thermoplastic
polymer material with the inorganic filler as defined in the
invention makes it possible to obtain an electrically insulating
layer having improved mechanical properties, notably in terms of
elongation at break and tensile strength, while guaranteeing good
thermal conductivity, or even better thermal conductivity.
[0019] A mixture of the inorganic fillers is preferably a mixture
of two or three of said inorganic fillers.
[0020] In the present invention, aluminium oxide, also commonly
known as "alumina", is a chemical compound of formula
Al.sub.2O.sub.3.
[0021] Hydrated aluminium oxide or hydrated alumina may be an
aluminium oxide monohydrate or polyhydrate, and preferably
monohydrate or trihydrate.
[0022] As examples of aluminium oxide monohydrate, we may mention
boehmite, which is the gamma polymorph of AlO(OH) or
Al.sub.2O.sub.3.H.sub.2O; or diaspore, which is the alpha polymorph
of AlO(OH) or Al.sub.2O.sub.3.H.sub.2O.
[0023] As examples of aluminium oxide polyhydrate, and preferably
trihydrate, we may mention gibbsite or hydrargillite, which is the
gamma polymorph of Al(OH).sub.3; bayerite, which is the alpha
polymorph of Al(OH).sub.3; or nordstrandite, which is the beta
polymorph of Al(OH).sub.3.
[0024] Hydrated aluminium oxide is also well known by the name
"aluminium oxide hydroxide" or "alumina hydroxide".
[0025] Aluminium oxide is preferred as the inorganic filler.
[0026] The aluminium oxide (or magnesium oxide) is notably a
calcined aluminium oxide (or a calcined magnesium oxide,
respectively).
[0027] In the present invention, the inorganic filler may represent
at least about 1 wt %, preferably at least about 2 wt %, especially
preferably at least about 5 wt %, and more especially preferably at
least about 10 wt %, relative to the total weight of the polymer
composition.
[0028] The inorganic filler preferably represents at most about 40
wt %, especially preferably at most about 30 wt %, and more
especially preferably at most about 25 wt %, relative to the total
weight of the polymer composition.
[0029] The inorganic filler may be in the form of particles ranging
in size from about 0.01 to 6 .mu.m, preferably from about 0.05 to 2
.mu.m, especially preferably from about 0.075 to 1.5 .mu.m, and
more especially preferably from about 0.1 to 1.1 .mu.m.
[0030] When considering several particles of inorganic filler
according to the invention, the term "size" signifies the
number-average size of the set of particles of a given population,
this size being determined conventionally by methods that are
familiar to a person skilled in the art.
[0031] The size of the particle or particles according to the
invention may be determined for example by microscopy, notably by
scanning electron microscope (SEM) or by transmission electron
microscope (TEM).
[0032] According to a preferred embodiment of the invention, the
inorganic filler may comprise a mixture of particles ranging in
size from about 0.01 to 0.50 .mu.m, and preferably from about 0.05
to 0.25 .mu.m; and of particles ranging in size from about 0.60 to
2 .mu.m, and preferably from about 0.75 to 1.5 .mu.m.
[0033] The inorganic filler may be "treated" or "untreated", and is
preferably "treated".
[0034] "Treated inorganic filler" means an inorganic filler that
has undergone a surface treatment, or in other words, a
surface-treated inorganic filler. Said surface treatment notably
allows the surface properties of the inorganic filler to be
modified, for example to improve the compatibility of the inorganic
filler with the thermoplastic polymer material.
[0035] In a preferred embodiment, the inorganic filler of the
invention may be silanized, or in other words may be treated to
obtain a silanized inorganic filler.
[0036] The surface treatment used for obtaining the silanized
inorganic filler is notably a surface treatment starting from at
least one silane compound (with or without coupling agent), this
type of surface treatment being familiar to a person skilled in the
art.
[0037] Thus, the silanized inorganic filler of the invention may
comprise siloxane and/or silane groups on its surface. Said groups
may be of the vinylsilane, alkylsilane, epoxysilane,
methacryloxysilane, acryloxysilane, aminosilane or mercaptosilane
type.
[0038] The silane compound used for obtaining the silanized
inorganic filler may be selected from: [0039]
alkyltrimethoxysilanes or alkyltriethoxysilanes, such as for
example octadecyltrimethoxysilane (OdTMS--C18),
octyl(triethoxy)silane (OTES--C8), methyl trimethoxysilane,
hexadecyl trimethoxysilane, [0040] vinyltrimethoxysilanes or
vinyltriethoxysilanes, [0041] methacryloxylsilanes or
acryloxysilanes, such as for example 3-methacryloxypropyl
methyldimethoxysilane, 3-methacryloxypropyl trimethoxysilane,
3-methacryloxypropyl trimethoxysilane, 3-acryloxypropyl
trimethoxysilane, and [0042] a mixture thereof.
[0043] The inorganic filler may have a specific surface area by the
BET method from about 1 to 20 g/cm.sup.2, and preferably from about
7.5 to 17 g/cm.sup.2.
[0044] In the present invention, the specific surface area of the
inorganic filler may easily be determined according to standard DIN
9277 (2010).
[0045] According to a preferred embodiment of the invention, the
inorganic filler may comprise a mixture of particles of specific
surface area from about 2 to 9 g/cm.sup.2, and particles of
specific surface area from about 10 to 17 g/cm.sup.2.
[0046] The polypropylene-based thermoplastic polymer material may
comprise a propylene homopolymer or copolymer P.sub.1, and
preferably a propylene copolymer P.sub.1.
[0047] The propylene homopolymer P.sub.1 preferably has an elastic
modulus from about 1250 to 1600 MPa.
[0048] The propylene homopolymer P.sub.1 may represent at least 10
wt %, and preferably from 15 to 30 wt %, relative to the total
weight of the polypropylene-based thermoplastic polymer
material.
[0049] As examples of propylene copolymers P.sub.1, we may mention
the copolymers of propylene and olefin, the olefin notably being
selected from ethylene and an .alpha..sub.1 olefin different from
propylene.
[0050] The ethylene or the .alpha..sub.1 olefin different from
propylene of the copolymer of propylene and olefin preferably
represents at most about 15 mol %, and especially preferably at
most about 10 mol %, relative to the total number of moles of
copolymer of propylene and olefin.
[0051] The .alpha..sub.1 olefin different from propylene may
correspond to the formula CH.sub.2.dbd.CH--R.sup.1, in which
R.sup.1 is a linear or branched alkyl group having from 2 to 12
carbon atoms, notably selected from the following olefins:
1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-octene,
1-decene, 1-dodecene, and a mixture thereof.
[0052] The copolymers of propylene and ethylene are preferred as
the propylene copolymer P.sub.1.
[0053] The propylene copolymer P.sub.1 may be a random propylene
copolymer or a heterophase propylene copolymer, and is preferably a
heterophase propylene copolymer.
[0054] In the invention, the random propylene copolymer P.sub.1
preferably has an elastic modulus from about 600 to 1200 MPa.
[0055] As an example of a random propylene copolymer P.sub.1, we
may mention that marketed by the company Borealis under the
reference Bormed.RTM. RB 845 MO.
[0056] The heterophase propylene copolymer P.sub.1 may comprise a
thermoplastic phase of the propylene type and a thermoplastic
elastomer phase of the ethylene/.alpha..sub.z olefin copolymer
type.
[0057] The .alpha..sub.2 olefin of the thermoplastic elastomer
phase of the heterophase propylene copolymer P.sub.1 may be
propylene.
[0058] The thermoplastic elastomer phase of the heterophase
propylene copolymer P.sub.1 may represent at least about 20 wt %,
and preferably at least about 45 wt %, relative to the total weight
of the heterophase propylene copolymer P.sub.1.
[0059] The heterophase propylene copolymer P.sub.1 preferably has
an elastic modulus from about 50 to 1200 MPa, and especially
preferably: either an elastic modulus from about 50 to 550 MPa, and
more especially preferably from about 50 to 300 MPa; or an elastic
modulus from about 600 to 1200 MPa.
[0060] As examples of heterophase propylene copolymer, we may
mention the heterophase propylene copolymer marketed by the company
LyondellBasell under the reference Adflex.RTM. Q 200 F, or the
heterophase copolymer marketed by the company LyondellBasell under
the reference EP.RTM. 2967.
[0061] The propylene homopolymer or copolymer P.sub.1 may have a
melting point above about 110.degree. C., preferably above about
130.degree. C., especially preferably above about 140.degree. C.,
and more especially preferably from about 140 to 170.degree. C.
[0062] The propylene homopolymer or copolymer P.sub.1 may have an
enthalpy of fusion from about 20 to 100 J/g.
[0063] The propylene homopolymer P.sub.1 may have an enthalpy of
fusion from about 80 to 90 J/g.
[0064] The random propylene copolymer P.sub.1 may have an enthalpy
of fusion from about 40 to 80 J/g.
[0065] The heterophase propylene copolymer P.sub.1 may have an
enthalpy of fusion from about 20 to 50 J/g.
[0066] The propylene homopolymer or copolymer P.sub.1 may have a
melt flow index from 0.5 to 3 g/10 min, measured at about
230.degree. C. with a load of about 2.16 kg according to standard
ASTM D1238-00.
[0067] The random propylene copolymer P.sub.1 may have a melt flow
index from 1.2 to 2.5 g/10 min, and preferably from 1.5 to 2.5 g/10
min, measured at about 230.degree. C. with a load of about 2.16 kg
according to standard ASTM D1238-00.
[0068] The heterophase propylene copolymer P.sub.1 may have a melt
flow index from 0.5 to 1.5 g/10 min, and preferably from about 0.5
to 1.4 g/10 min, measured at about 230.degree. C. with a load of
about 2.16 kg according to standard ASTM D1238-00.
[0069] The polypropylene-based thermoplastic polymer material may
comprise several different propylene copolymers P.sub.1, notably
two different propylene copolymers P.sub.1, said propylene
copolymers P.sub.1 being as defined above.
[0070] In particular, the polypropylene-based thermoplastic polymer
material may comprise a random propylene copolymer (as first
propylene copolymer P.sub.1) and a heterophase propylene copolymer
(as second propylene copolymer P.sub.1), or two different
heterophase propylene copolymers.
[0071] When the polypropylene-based thermoplastic polymer material
comprises a random propylene copolymer and a heterophase propylene
copolymer, said heterophase propylene copolymer preferably has an
elastic modulus from about 50 to 300 MPa.
[0072] According to one embodiment of the invention, the two
heterophase propylene copolymers have a different elastic modulus.
Preferably, the polypropylene-based thermoplastic polymer material
comprises a first heterophase propylene copolymer having an elastic
modulus from about 50 to 550 MPa, and especially preferably from
about 50 to 300 MPa; and a second heterophase propylene copolymer
having an elastic modulus from about 600 to 1200 MPa.
[0073] Advantageously, the first and second heterophase propylene
copolymers have a melt flow index as defined in the invention.
[0074] These combinations of propylene copolymers P.sub.1 may
advantageously make it possible to improve the mechanical
properties of the polymer layer. In particular, the combination
makes it possible to obtain optimized mechanical properties of the
polymer layer, notably in terms of elongation at break, and
flexibility; and/or makes it possible to form a more homogeneous
polymer layer, and notably promotes dispersion of the dielectric
liquid in the polypropylene-based thermoplastic polymer material of
said polymer layer.
[0075] According to a preferred embodiment of the invention, the
propylene copolymer P.sub.1 or the propylene copolymers P.sub.1
when there are several of them, represent(s) at least about 50 wt
%, preferably from about 55 to 90 wt %, and especially preferably
from about 60 to 90 wt %, relative to the total weight of the
polypropylene-based thermoplastic polymer material.
[0076] The random propylene copolymer P.sub.1 may represent at
least 20 wt %, and preferably from 30 to 70 wt %, relative to the
total weight of the polypropylene-based thermoplastic polymer
material.
[0077] The heterophase propylene copolymer P.sub.1 or the
heterophase propylene copolymers P.sub.1 when there are several of
them, may represent from about 5 to 95 wt %, preferably from about
50 to 90 wt %, and especially preferably from about 60 to 80 wt %,
relative to the total weight of the polypropylene-based
thermoplastic polymer material.
[0078] The polypropylene-based thermoplastic polymer material may
further comprise an olefin homopolymer or copolymer P.sub.2.
[0079] Said olefin homopolymer or copolymer P.sub.2 is preferably
different from said propylene homopolymer or copolymer P.sub.1.
[0080] The olefin of the olefin copolymer P.sub.2 may be selected
from ethylene and an .alpha..sub.3 olefin corresponding to the
formula CH.sub.2.dbd.CH--R.sup.2, in which R.sup.2 is a linear or
branched alkyl group having from 1 to 12 carbon atoms.
[0081] The .alpha..sub.3 olefin is preferably selected from the
following olefins: propylene, 1-butene, isobutylene, 1-pentene,
4-methyl-1-pentene, 1-hexene, 1-octene, 1-decene, 1-dodecene, and a
mixture thereof.
[0082] The .alpha..sub.3 olefin of the propylene, 1-hexene or
1-octene type is particularly preferred.
[0083] The combination of polymers P.sub.1 and P.sub.2 makes it
possible to obtain a thermoplastic polymer material having good
mechanical properties, notably in terms of elastic modulus, and
electrical properties.
[0084] The olefin homopolymer or copolymer P.sub.2 is preferably an
ethylene polymer.
[0085] According to a preferred embodiment of the invention, the
ethylene polymer is a low-density polyethylene, a low-density
linear polyethylene, a medium-density polyethylene, or a
high-density polyethylene, and preferably a high-density
polyethylene; notably according to standard ISO 1183A (at a
temperature of 23.degree. C.).
[0086] The ethylene polymer preferably has an elastic modulus of at
least 400 MPa, and especially preferably at least 500 MPa.
[0087] In the present invention, the elastic modulus or Young's
modulus of a polymer (known by the English term "Tensile Modulus")
is well known by a person skilled in the art, and may easily be
determined according to standard ISO 527-1, -2 (2012). Standard ISO
527 has a first part, designated "ISO 527-1", and a second part,
designated "ISO 527-2" specifying the test conditions relating to
the general principles of the first part of standard ISO 527.
[0088] In the present invention, the expression "low-density"
signifies having a density from about 0.91 to 0.925 g/cm.sup.3,
said density being measured according to standard ISO 1183A (at a
temperature of 23.degree. C.).
[0089] In the present invention, the expression "medium-density"
signifies having a density from about 0.926 to 0.940 g/cm.sup.3,
said density being measured according to standard ISO 1183A (at a
temperature of 23.degree. C.).
[0090] In the present invention, the expression "high-density"
signifies having a density from 0.941 to 0.965 g/cm.sup.3, said
density being measured according to standard ISO 1183A (at a
temperature of 23.degree. C.).
[0091] According to a preferred embodiment of the invention, the
olefin homopolymer or copolymer P.sub.2 represents from about 5 to
50 wt %, and especially preferably from about 10 to 40 wt %,
relative to the total weight of the polypropylene-based
thermoplastic polymer material.
[0092] According to an especially preferred embodiment of the
invention, the polypropylene-based thermoplastic polymer material
comprises two propylene copolymers P.sub.1 such as a random
propylene copolymer and a heterophase propylene copolymer or two
different heterophase propylene copolymers; and an olefin
homopolymer or copolymer P.sub.2 such as an ethylene polymer. This
combination of propylene copolymers P.sub.1 and of an olefin
homopolymer or copolymer P.sub.2 offers the possibility of further
improvement of the mechanical properties of the polymer layer,
while guaranteeing good thermal conductivity.
[0093] The thermoplastic polymer material of the polymer
composition of the electrically insulating layer of the cable of
the invention is preferably heterophase (i.e. it comprises several
phases). The presence of several phases generally results from
mixing two different polyolefins, such as a mixture of different
propylene polymers or a mixture of a propylene polymer and an
ethylene polymer.
[0094] The polymer composition of the invention may further
comprise a dielectric liquid, notably forming an intimate mixture
with the thermoplastic polymer material.
[0095] The dielectric liquid improves the interface inorganic
filler/polypropylene-based thermoplastic polymer material. The
presence of the dielectric liquid enables to obtain better
dielectric properties (i.e. better electrical insulation), and
notably better dielectric strength of the layer obtained from the
polymer composition. It can also allow improving mechanical
properties and/or ageing resistance of said layer.
[0096] As examples of dielectric liquid, we may mention mineral
oils (e.g. naphthenic oils, paraffinic oils or aromatic oils),
vegetable oils (e.g. soya oil, linseed oil, colza oil, maize oil or
castor oil) or synthetic oils such as aromatic hydrocarbons
(alkylbenzenes, alkylnaphthalenes, alkylbiphenyls,
alkydiarylethylenes, etc.), silicone oils, ether oxides, organic
esters or aliphatic hydrocarbons, or mixtures thereof.
[0097] The dielectric liquid preferably comprises at least one
mineral oil.
[0098] According to a particular embodiment, the dielectric liquid
represents from about 1 to 20 wt %, preferably from about 2 to 15
wt %, and especially preferably from about 3 to 12 wt %, relative
to the total weight of the polypropylene-based thermoplastic
polymer material.
[0099] The dielectric liquid may comprise at least about 70 wt % of
mineral oil, and preferably at least about 80 wt % of mineral oil,
relative to the total weight of the dielectric liquid.
[0100] The mineral oil is generally liquid at about 20-25.degree.
C.
[0101] The mineral oil is preferably selected from naphthenic oils
and paraffinic oils.
[0102] The mineral oil is obtained from the refining of a petroleum
crude.
[0103] According to an especially preferred embodiment of the
invention, the mineral oil comprises a content of paraffinic carbon
(cP) from about 45 to 65 at %, a content of naphthenic carbon (nC)
from about 35 to 55 at % and a content of aromatic carbon (aC) from
about 0.5 to 10 at %.
[0104] The dielectric liquid advantageously comprises a mineral
oil, notably as defined in the invention, and at least one polar
compound of the type benzophenone, acetophenone or a derivative
thereof.
[0105] In a particular embodiment, the polar compound of the type
benzophenone, acetophenone or a derivative thereof represents at
least about 2.5 wt %, preferably at least about 3.5 wt %, and
especially preferably at least about 4 wt %, relative to the total
weight of the dielectric liquid.
[0106] The dielectric liquid may comprise at most about 30 wt %,
preferably at most about 20 wt %, and even more preferably at most
about 15 wt %, of polar compound of the type benzophenone,
acetophenone or a derivative thereof, relative to the total weight
of the dielectric liquid. This maximum amount makes it possible to
guarantee moderate, or even low (e.g. below about 10.sup.-3)
dielectric losses, as well as prevent migration of the dielectric
liquid out of the electrically insulating layer.
[0107] According to a preferred embodiment of the invention, the
polar compound of the type benzophenone, acetophenone or a
derivative thereof is selected from benzophenone, dibenzosuberone,
fluorenone and anthrone. Benzophenone is particularly
preferred.
[0108] The polypropylene-based thermoplastic polymer material may
further comprise one or more additives.
[0109] The additives are well known by a person skilled in the art
and may be selected from agents favouring application, such as
lubricants, compatibilizers, or coupling agents, antioxidants,
anti-UV agents, anti-copper agents, anti-water treeing agents,
pigments, and a mixture thereof.
[0110] The polypropylene-based thermoplastic polymer material may
typically comprise from about 0.01 to 5 wt %, and preferably from
about 0.1 to 2 wt % of additives, relative to the total weight of
the polypropylene-based thermoplastic polymer material.
[0111] More particularly, the antioxidants make it possible to
protect the polymer composition from the thermal stresses generated
during the steps of manufacture of the cable or operation of the
cable.
[0112] The antioxidants are preferably selected from hindered
phenols, thioesters, sulphur-based antioxidants, phosphorus-based
antioxidants, antioxidants of the amine type, and a mixture
thereof.
[0113] As examples of hindered phenols, we may mention
1,2-bis(3,5-di-tert-butyl-4-hydroxyhydrocinnamoyl)hydrazine
(Irganox.RTM. MD 1024), pentaerythritol
tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate)
(Irganox.RTM. 1010), octadecyl
3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate (Irganox.RTM.
1076),
1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene
(Irganox.RTM. 1330), 4,6-bis(octylthiomethyl)-o-cresol
(Irgastab.RTM. KV10 or Irganox.RTM. 1520),
2,2'-thiobis(6-tert-butyl-4-methylphenol) (Irganox.RTM. 1081),
2,2'-thiodiethylene
bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] (Irganox.RTM.
1035), tris(3,5-di-tert-butyl-4-hydroxybenzyl)isocyanurate
(Irganox.RTM. 3114),
2,2'-oxamido-bis(ethyl-3(3,5-di-tert-butyl-4-hydroxyphenyl)propionate)
(Naugard XL-1), or
2,2'-methylenebis(6-tert-butyl-4-methylphenol).
[0114] As examples of sulphur-based antioxidants, we may mention
thioethers such as didodecyl-3,3'-thiodipropionate (Irganox.RTM.
PS800), distearyl thiodipropionate or
dioctadecyl-3,3'-thiodipropionate (Irganox.RTM. PS802),
bis[2-methyl-4-{3-n-alkyl (C.sub.12 or C.sub.14)
thiopropionyloxy}-5-tert-butylphenyl]sulphide,
thiobis-[2-tert-butyl-5-methyl-4,1-phenylene] bis
[3-(dodecylthio)propionate], or 4,6-bis(octylthiomethyl)-o-cresol
(Irganox.RTM. 1520 or Irgastab.RTM. KV10).
[0115] As examples of phosphorus-based antioxidants, we may mention
tris(2,4-di-tert-butylphenyl)phosphite (Irgafos.RTM. 168) or
bis(2,4-di-tert-butylphenyl)pentaerythritol diphosphite
(Ultranox.RTM. 626).
[0116] As examples of antioxidants of the amine type, we may
mention phenylene diamines (e.g. paraphenylene diamines such as
1PPD or 6PPD), diphenylamine styrene, diphenylamines,
4-(1-methyl-1-phenylethyl)-N-[4-(1-methyl-1-phenylethyl)phenyl]aniline
(Naugard 445), mercapto benzimidazoles, or polymerized
2,2,4-trimethyl-1,2-dihydroquinoline (TMQ).
[0117] As examples of mixtures of antioxidants usable according to
the invention, we may mention Irganox B 225, which comprises an
equimolar mixture of Irgafos 168 and Irganox 1010 as described
above.
[0118] The polymer composition of the electrically insulating layer
of the invention is a thermoplastic polymer composition. Therefore
it is not crosslinkable.
[0119] In particular, the polymer composition does not comprise
crosslinking agents, coupling agents of the silane type, peroxides
and/or additives that allow crosslinking. In fact such agents
degrade the polypropylene-based thermoplastic polymer material.
[0120] The polymer composition is preferably recyclable.
[0121] The polymer composition may further comprise at least one
inorganic filler different from the inorganic filler selected from
aluminium oxide, a hydrated aluminium oxide, magnesium oxide, zinc
oxide, and a mixture thereof, such as talc, aluminium trihydrate
Al(OH).sub.3, or magnesium dihydrate Mg(OH).sub.2; and/or at least
one halogen-free mineral filler intended to improve the fire
behaviour of the polymer composition.
[0122] The inorganic filler different from the inorganic filler
selected from aluminium oxide, a hydrated aluminium oxide,
magnesium oxide, zinc oxide, and a mixture thereof, and/or the
halogen-free mineral filler may represent at most about 30 wt %,
preferably at most about 20 wt %, especially preferably at most
about 10 wt %, and more especially preferably at most about 5 wt %,
relative to the total weight of the polymer composition.
[0123] In order to guarantee a so-called "HFFR" electric cable
("Halogen-Free Flame Retardant"), the cable of the invention
preferably does not comprise halogenated compounds. These
halogenated compounds may be of all kinds, such as for example
fluorinated polymers or chlorinated polymers such as polyvinyl
chloride (PVC), halogenated plasticizers, halogenated mineral
fillers, etc.
[0124] The polymer composition may be prepared by mixing the
polypropylene-based thermoplastic polymer material with at least
one inorganic filler selected from aluminium oxide, a hydrated
aluminium oxide, magnesium oxide, zinc oxide, and a mixture
thereof, optionally a dielectric liquid and optionally one or more
additives as defined in the invention.
[0125] The electrically insulating layer of the cable of the
invention is a non-crosslinked layer or in other words a
thermoplastic layer.
[0126] In the invention, the expression "non-crosslinked layer" or
"thermoplastic layer" signifies a layer whose gel content according
to standard ASTM D2765-01 (xylene extraction) is at most about 30%,
preferably at most about 20%, especially preferably at most about
10%, more particularly preferably at most 5%, and even more
especially preferably 0%.
[0127] In one embodiment of the invention, the electrically
insulating layer, preferably non-crosslinked, has a thermal
conductivity of at least 0.30 W/mK at 40.degree. C., preferably at
least 0.31 W/mK at 40.degree. C., especially preferably at least
0.32 W/mK at 40.degree. C., more especially preferably at least
0.33 W/mK at 40.degree. C., even more especially preferably at
least 0.34 W/mK at 40.degree. C., and even more especially
preferably at least 0.35 W/mK at 40.degree. C.
[0128] The thermal conductivity is preferably measured by the
method that is well known by the English term "Transient Plane
Source or TPS". Advantageously, the thermal conductivity is
measured using an instrument marketed under the reference HOT DISK
TPS 2500S by the company THERMOCONCEPT.
[0129] In a particular embodiment, the electrically insulating
layer, preferably non-crosslinked, has a tensile strength (TS) of
at least 8.5 MPa, preferably at least about 10 MPa, and especially
preferably at least about 15 MPa, before ageing (according to
standard CEI 20-86).
[0130] In a particular embodiment, the electrically insulating
layer, preferably non-crosslinked, has an elongation at break (EB)
of at least about 250%, preferably at least about 300%, and
especially preferably at least about 350%, before ageing (according
to standard CEI 20-86).
[0131] In a particular embodiment, the electrically insulating
layer, preferably non-crosslinked, has a tensile strength (TS) of
at least 8.5 MPa, preferably at least about 10 MPa, and especially
preferably of at least about 15 MPa, after ageing (according to
standard CEI 20-86).
[0132] In a particular embodiment, the electrically insulating
layer, preferably non-crosslinked, has an elongation at break (EB)
of at least about 250%, preferably at least about 300%, and
especially preferably at least about 350%, after ageing (according
to standard CEI 20-86).
[0133] The tensile strength (TS) and the elongation at break (EB)
(before or after ageing) may be determined according to standard NF
EN 60811-1-1, notably using an instrument marketed under the
reference 3345 by the company Instron.
[0134] Ageing is generally carried out at 135.degree. C. for 240
hours (or 10 days).
[0135] The electrically insulating layer of the cable of the
invention is preferably a recyclable layer.
[0136] The electrically insulating layer of the invention may be an
extruded layer, notably by methods well known by a person skilled
in the art.
[0137] The electrically insulating layer has a variable thickness
depending on the type of cable envisaged. In particular, when the
cable according to the invention is a medium-voltage cable, the
thickness of the electrically insulating layer is typically from
about 4 to 5.5 mm, and more particularly about 4.5 mm. When the
cable according to the invention is a high-voltage cable, the
thickness of the electrically insulating layer typically varies
from 17 to 18 mm (for voltages of the order of about 150 kV) and up
to thicknesses from about 20 to 25 mm for voltages above 150 kV
(high-voltage cables). The aforementioned thicknesses depend on the
size of the elongated electrically conducting element.
[0138] In the present invention, "electrically insulating layer"
means a layer whose electrical conductivity may be of at most
1.10.sup.-8 S/m (siemens per metre), preferably at most 1.10.sup.-9
S/m, and especially preferably at most 1.10.sup.-10 S/m, measured
at about 25.degree. C. with direct current.
[0139] The polymer composition may then comprise less than about 6
wt % of electrically conductive filler, preferably less than about
1 wt % of electrically conductive filler, and especially preferably
about 0 wt % of electrically conductive filler, relative to the
total weight of the polymer composition.
[0140] The electrically conductive filler may be selected from
carbon blacks, graphites, and a mixture thereof.
[0141] The electrically insulating layer of the invention may
comprise at least one polypropylene-based thermoplastic polymer
material, at least one inorganic filler selected from aluminium
oxide, a hydrated aluminium oxide, magnesium oxide, zinc oxide, and
a mixture thereof, one or more additives, optionally at least one
inorganic filler different from the inorganic filler selected from
aluminium oxide, a hydrated aluminium oxide, magnesium oxide, zinc
oxide, and a mixture thereof, and optionally at least one
halogen-free mineral filler intended to improve the fire behaviour
of the polymer composition, the aforementioned ingredients being as
defined in the invention.
[0142] The proportions of the various ingredients in the
electrically insulating layer may be identical to those as
described in the invention for these same ingredients in the
polymer composition.
[0143] The cable of the invention relates more particularly to the
field of electric cables operating with direct current (DC) or with
alternating current (AC).
[0144] The electrically insulating layer of the invention may
surround the elongated electrically conducting element.
[0145] The elongated electrically conducting element is preferably
located at the centre of the cable.
[0146] The elongated electrically conducting element may be a
single-core conductor such as for example a metal wire or a
multicore conductor such as a plurality of metal wires, twisted or
not.
[0147] The elongated electrically conducting element may be of
aluminium, aluminium alloy, copper, copper alloy, or a combination
thereof.
[0148] According to a preferred embodiment of the invention, the
electric cable comprises: [0149] at least one semiconducting layer
surrounding the elongated electrically conducting element, and
[0150] an electrically insulating layer as defined in the
invention.
[0151] The electrically insulating layer has more particularly an
electrical conductivity lower than that of the semiconducting
layer. More particularly, the electrical conductivity of the
semiconducting layer may be at least 10 times greater than the
electrical conductivity of the electrically insulating layer,
preferably at least 100 times greater than the electrical
conductivity of the electrically insulating layer, and especially
preferably at least 1000 times greater than the electrical
conductivity of the electrically insulating layer.
[0152] The semiconducting layer may surround the electrically
insulating layer. The semiconducting layer may then be an external
semiconducting layer.
[0153] The electrically insulating layer may surround the
semiconducting layer.
[0154] The semiconducting layer may then be an internal
semiconducting layer.
[0155] The semiconducting layer is preferably an internal
semiconducting layer.
[0156] The electric cable of the invention may further comprise
another semiconducting layer.
[0157] Thus, in this embodiment, the cable of the invention may
comprise: [0158] at least one elongated electrically conducting
element, preferably located at the centre of the cable, [0159] a
first semiconducting layer surrounding the elongated electrically
conducting element, [0160] an electrically insulating layer
surrounding the first semiconducting layer, and [0161] a second
semiconducting layer surrounding the electrically insulating
layer,
[0162] the electrically insulating layer being as defined in the
invention.
[0163] In the present invention, "semiconducting layer" means a
layer whose electrical conductivity may be strictly above
1.10.sup.-8 S/m (siemens per metre), preferably at least
1.10.sup.-3 S/m, and preferably may be below 1.10.sup.3 S/m,
measured at 25.degree. C. in direct current.
[0164] In a particular embodiment, the first semiconducting layer,
the electrically insulating layer and the second semiconducting
layer make up a three-layer insulation. In other words, the
electrically insulating layer is in direct physical contact with
the first semiconducting layer, and the second semiconducting layer
is in direct physical contact with the electrically insulating
layer.
[0165] The first semiconducting layer (or, respectively, the second
semiconducting layer) is preferably obtained from a polymer
composition comprising at least one polypropylene-based
thermoplastic polymer material as defined in the invention, and
optionally at least one electrically conductive filler as defined
in the invention.
[0166] The electrically conductive filler preferably represents a
sufficient amount for the layer to be semiconducting.
[0167] Preferably, the polymer composition may comprise at least
about 6 wt % of electrically conductive filler, preferably at least
about 10 wt % of electrically conductive filler, preferably at
least about 15 wt % of electrically conductive filler, and even
more preferably at least about 25 wt % of electrically conductive
filler, relative to the total weight of the polymer
composition.
[0168] The polymer composition may comprise at most about 45 wt %
of electrically conductive filler, and preferably at most about 40
wt % of electrically conductive filler, relative to the total
weight of the polymer composition.
[0169] The first semiconducting layer (or, respectively, the second
semiconducting layer) is preferably a thermoplastic layer or a
non-crosslinked layer.
[0170] The cable may further comprise an outer protective sheath
surrounding the electrically insulating layer (or the second
semiconducting layer if present).
[0171] The outer protective sheath may be in direct physical
contact with the electrically insulating layer (or the second
semiconducting layer if present).
[0172] The outer protective sheath may be an electrically
insulating sheath.
[0173] The electric cable may further comprise an electrical screen
(e.g. metallic) surrounding the second semiconducting layer. In
this case, the electrically insulating sheath surrounds said
electrical screen and the electrical screen is between the
electrically insulating sheath and the second semiconducting
layer.
[0174] This metal screen may be a so-called "wire" screen made up
of an assembly of copper or aluminium conductors arranged around
and along the second semiconducting layer, a so-called "taped"
screen made up of one or more copper or aluminium conductive metal
tapes optionally placed helically around the second semiconducting
layer or an aluminium conductive metal tape placed longitudinally
around the second semiconducting layer and made impervious with
glue in the zones where parts of said tape overlap, or a so-called
"impervious" screen of the metal tube type optionally consisting of
lead or lead alloy and surrounding the second semiconducting layer.
This last-mentioned type of screen notably provides a barrier to
moisture, which tends to penetrate the electric cable in the radial
direction.
[0175] The metal screen of the electric cable of the invention may
comprise a so-called "wire" screen and a so-called "impervious"
screen or a so-called "wire" screen and a so-called "taped"
screen.
[0176] All the types of metal screens may perform the role of
earthing the electric cable and may thus carry away fault currents,
for example in the case of short-circuit in the network in
question.
[0177] Other layers, such as layers that swell in the presence of
moisture, may be added between the second semiconducting layer and
the metal screen, these layers ensuring longitudinal imperviousness
of the electric cable to water.
[0178] The second object of the invention is a method of
manufacturing an electric cable according to the first object of
the invention, characterized in that it comprises at least one step
1) of extrusion of a polymer composition as defined in the first
object of the invention around an elongated electrically conducting
element, to obtain an electrically insulating layer (extruded)
surrounding said elongated electrically conducting element.
[0179] Step 1) may be carried out by techniques familiar to a
person skilled in the art, for example using an extruder.
[0180] In step 1), the composition leaving the extruder is called
"non-crosslinked", the temperature as well as the residence time
within the extruder being optimized in consequence.
[0181] At the outlet of the extruder, an extruded layer is
therefore obtained around said electrically conducting element,
which may or may not be in direct physical contact with said
elongated electrically conducting element.
[0182] The method preferably does not comprise a step of
crosslinking the layer obtained in step 1).
[0183] The electrically insulating layer and/or the semiconducting
layer or layers of the electric cable of the invention may be
obtained by successive extrusion or by co-extrusion.
[0184] Prior to extrusion of each of these layers around at least
one elongated electrically conducting element, all of the
constituents required for formation of each of these layers may be
metered and mixed in a continuous mixer of the BUSS co-kneader
type, twin-screw extruder or some other type of mixer suitable for
polymer mixtures, notably with fillers. The mixture may then be
extruded in the form of rods, and then cooled and dried to be
granulated, or the mixture may be granulated directly, by
techniques familiar to a person skilled in the art. The granules
may then be fed into a single-screw extruder in order to extrude
and deposit the composition around the elongated electrically
conducting element to form the layer in question.
[0185] The various compositions may be extruded one after another
to surround the elongated electrically conducting element
successively, and thus form the various layers of the electric
cable of the invention.
[0186] They may alternatively be extruded concomitantly by
co-extrusion using a single extruder head, co-extrusion being a
method that is well known by a person skilled in the art.
[0187] Whether in the step of formation of the granules or in the
step of extrusion on the cable, the operating conditions are
familiar to a person skilled in the art. Notably, the temperature
inside the mixing or extrusion device may be above the melting
point of the predominant polymer or of the polymer having the
highest melting point, among the polymers used in the composition
to be employed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0188] FIG. 1 shows an electric cable in accordance with one
embodiment.
DETAILED DESCRIPTION
Examples
[0189] FIG. 1 shows a schematic view of an electric cable according
to a preferred embodiment of the invention.
[0190] For reasons of clarity, only the elements essential for
understanding the invention have been shown schematically, and they
are not drawn to scale.
[0191] The medium-voltage or high-voltage electric cable 1
according to the first object of the invention, illustrated in FIG.
1, comprises a central elongated electrically conducting element 2,
notably made of copper or aluminium. The electric cable 1 further
comprises several layers arranged successively and coaxially around
this central elongated electrically conducting element 2, namely: a
first semiconducting layer 3 called "internal semiconducting
layer", an electrically insulating layer 4, a second semiconducting
layer 5 called "external semiconducting layer", a metal screen 6
for earthing and/or protection, and an outer protective sheath
7.
[0192] The electrically insulating layer 4 is a non-crosslinked
extruded layer, obtained from the polymer composition as defined in
the invention.
[0193] The semiconducting layers 3 and 5 are thermoplastic (i.e.
non-crosslinked) extruded layers.
[0194] The presence of the metal screen 6 and outer protective
sheath 7 is preferred, but not essential, this cable structure
being well known per se by a person skilled in the art.
[0195] Polymer Compositions
[0196] A composition I1 according to the invention, i.e. comprising
at least one polypropylene-based thermoplastic polymer material and
at least aluminium oxide as inorganic filler, was compared against
a comparative composition C1, the composition C1 corresponding to a
composition comprising a polypropylene-based thermoplastic polymer
material identical to that used for the composition of the
invention I1 but comprising kaolin as inorganic filler, instead of
aluminium oxide.
[0197] Table 1 below presents polymer compositions, with the
amounts of the compounds expressed in percentages by weight,
relative to the total weight of the polymer composition.
TABLE-US-00001 TABLE 1 Polymer compositions C1 (*) I1 Heterophase
propylene copolymer 15 29 Random propylene copolymer 44 29
High-density polyethylene 21.5 21.5 Inorganic filler: kaolin 15 0
Inorganic filler: aluminium oxide 0 15 Dielectric liquid 4.5 4.5
Antioxidant 1.0 1.0 (*) Comparative composition not forming part of
the invention
[0198] The origin of the compounds in Table 1 is as follows: [0199]
high-density polyethylene marketed under the reference Eltex
A4009MFN1325 by the company Ineos and the density of which is 0.960
g/cm.sup.3 according to standard ISO 1183A at a temperature of
23.degree. C. (MFI=0.9), and the elastic modulus is 1700 MPa;
[0200] heterophase propylene copolymer marketed by the company
Basell Polyolefins under the reference Adflex.RTM. Q 200F; [0201]
random propylene copolymer marketed by the company Borealis under
the reference Bormed.RTM. RB 845 MO; [0202] dielectric liquid
comprising 95 wt % of a mineral oil marketed by the company Nynas
under the reference Nytex 810, and 5 wt % of benzophenone marketed
by the company Sigma-Aldrich under the reference B9300, relative to
the total weight of the dielectric liquid; [0203] antioxidant
marketed by the company Ciba under the reference Irganox B 225,
which comprises an equimolar mixture of Irgafos 168 and Irganox
1010; [0204] chalk as inorganic filler marketed under the reference
Omya EXH1, and [0205] aluminium oxide as inorganic filler marketed
under the reference P122SB or Timal-12.
[0206] 2. Preparation of the Non-Crosslinked Layers
[0207] The compositions presented in Table 1 are used as
follows.
[0208] The following constituents: mineral oil, antioxidant and
benzophenone of compositions C1 and I1 referred to in Table 1, for
each layer to be considered, are metered and mixed with stirring at
about 75.degree. C., to form a liquid mixture comprising the
dielectric liquid.
[0209] The liquid mixture is then mixed with the following
constituents: heterophase propylene copolymer, random propylene
copolymer, high-density polyethylene compositions C1 and I1
referred to in Table 1, for each polymer layer to be considered, in
a vessel. Then the resultant mixture and the inorganic filler, for
each polymer layer to be considered, are homogenized using a
twin-screw extruder ("Berstorff twin screw extruder") at a
temperature of about 145 to 180.degree. C., and then melted at
about 200.degree. C. (screw speed: 80 rev/min).
[0210] The homogenized and melted mixture is then granulated.
[0211] The granules were then pressed hot to form layers in the
form of plates.
[0212] Each of the polymer compositions C1 and I1 was prepared in
this way in the form of layers with a thickness of 1 mm for
evaluating their mechanical properties as well as layers with a
thickness of 8 mm for carrying out the measurements of thermal
conductivity.
[0213] These compositions C1 and I1 were then compared from the
standpoint of their mechanical properties (tensile
strength/elongation at break before and after ageing at 135.degree.
C. for 240 hours) and their thermal conductivity.
[0214] The tests of tensile strength (TS) and elongation at break
(EB) were carried out on the materials according to standard NF EN
60811-1-1, using an instrument marketed under the reference 3345 by
the company Instron.
[0215] The results corresponding to each of these tests are
reported in Table 2 (mechanical properties) below:
TABLE-US-00002 TABLE 2 Properties C1 (*) I1 TS (MPa) 13.5 19.5 EB
(%) 445 692 TS after ageing (MPa) 15.1 19.2 EB after ageing (%)
372.75 613.93 (*) Comparative composition not forming part of the
invention
[0216] All these results show that incorporating an inorganic
filler as defined in the invention in a polypropylene matrix
improves the mechanical properties of the electrically insulating
layer, notably in terms of tensile strength and elongation at
break, including after ageing.
[0217] The tests of thermal conductivity were carried out on the
materials according to the familiar method known by the English
term "Transient Plane Source or TPS", using an instrument marketed
under the reference HOT DISK TPS 2500S by the company
THERMOCONCEPT.
[0218] The results corresponding to these tests are reported in
Table 3 (thermal conductivity) below:
TABLE-US-00003 TABLE 3 Properties C1 (*) I1 Thermal conductivity
0.28 0.31 at 40.degree. C. (W/m K) (*) Comparative composition not
forming part of the invention
[0219] The results for thermal conductivity show that the presence
of an inorganic filler as defined in the invention in a
polypropylene matrix leads to an electrically insulating layer
having a thermal conductivity greater than that of an electrically
insulating layer in which the inorganic filler is chalk.
* * * * *